The aforementioned patent describes a method of recovering molybdenum oxide by oxidation of a molybdenum sulfide aqueous concentrate slurry containing impurities and in which the molybdenum concentrate has an average particle or grain size of 20 microns to 90 microns, preferably about 70 microns.
In this method, the contaminated molybdenum sulfide concentrate is slurried in the slurrying stage with water to form an aqueous suspension which is introduced into an autoclave and subjected to an oxidation process at elevated temperature and with an elevated oxygen partial pressure. In the reaction, the molybdenum sulfide is transformed into molybdenum oxide and sulfuric acid by a reaction scheme outlined in the aforementioned patent. The reacted suspension is discharged from the autoclave and the molybdenum oxide is filtered from the reacted suspension and the first filtrate which is thus obtained and which contains sulfuric acid, is neutralized with lime or calcium carbonate to form gypsum (calcium sulfate dihydrate), whereupon the suspension is filtered to remove the gypsum therefrom and the resulting second filtrate is recirculated to the slurrying stage.
It will be understood that the neutralization to form gypsum, to be strict, is only a partial neutralization and further that the molybdenum sulfide concentrate is prepared and introduced in consideration of the capacity of the apparatus and the amount of molybdenum oxide to be generated.
The throughput of the molybdenum compounds is thus dependent upon the size and configuration of the apparatus.
Make-up water is introduced to cover amounts which may be lost in the recovered molybdenum oxide and gypsum.
In an earlier process described in German Patent No. 28 30 394, not only is the second filtrate recirculated but a two-stage recirculation is provided. In a first recirculation stage, the reacted suspension after oxidation and before filtering off the molybdenum oxide is recirculated to the slurrying stage and is combined with an additional quantity of molybdenum sulfide concentrate before the mixture of the recycled suspension and additional molybdenum sulfide concentrate is returned to the autoclave.
This recirculation continues until the sulfuric acid content of the slurry is about 80 to 120 g/l. Only then is the molybdenum oxide filtered from the multiple recirculated suspension.
The second recirculation stage recirculates a product following the precipitation of gypsum. More particularly, the first filtrate obtained from filtering off molybdenum oxide is neutralized with the formation of calcium sulfate to a pH value in the range of 0.9 to 1.5 and preferably close to 0.9. The gypsum which is thereby produced is filtered off leaving a second filtrate which is recirculated to the slurrying stage and therefore combined with additional quantities of molybdenum sulfide concentrate before the resulting mixture is again fed to the autoclave. This double recirculation continues until the impurity elements in the second filtrate build up to a sufficiently high level as to warrant their recovery and restarting of the process.
In this earlier system, moreover, the impurity-enriched second filtrate is neutralized to a pH of about 2.5 with an alkali hydroxide and this second filtrate is returned to the autoclave where it is again subjected to oxidation with oxygen to precipitate out iron molybdate. The iron molybdate is filtered off and the filtrate is then subjected to further processing to recover the various impurity elements contained therein.
The latter process has been found to be highly satisfactory because the recirculation generates a sufficiently high level of impurities that such impurities can be readily recovered. The first recirculation which involves the recirculation of molybdenum oxide contributes to the avoidance of encrustation in the autoclave. The recycled filtrate is sufficiently hot so that external heat need not be provided and contributes to the maintenance of the otherwise exothermic reaction in the autoclave (MoS.sub.2 +9O.sub.2 +2H.sub.2 O=MoO.sub.3 +2H.sub.2 SO.sub.4).
In this system the suspension has a suspension density of about 50 to 75 g/l.
However, with this system, because of the multiple recycling operations, an apparatus for producing a given quantity of molybdenum oxide must be considerably larger than is desired and the energy which must be introduced, in spite of the exothermicity of the reaction, because of the recirculation of dense fluids, is comparatively large.
In order to avoid these disadvantages, in our aforementioned U.S. patent we have provided a method in which for an apparatus of a given size a particularly high throughput can be obtained, i.e. the efficiency of the apparatus is far more pronounced. Consequently, for a given output, the apparatus can be simplified and the energy input can be reduced.
In our prior patent, this is obtained by operating the oxidation stage with a suspension whose suspension density is in the range of 100 to 150 g/l and such that only the second filtrate obtained by neutralization of the first filtrate and filtering or separation of gypsum therefrom is recirculated. Important to the invention there described is that the second filtrate be recycled in such quantity that the suspension density of the suspension fed to the autoclave is maintained in the aforementioned range of 100 to 150 g/l.
Experience with our earlier system, however, has shown that in practice the composition of the molybdenum sulfide concentrate varies with time, e.g. by changes in the gangue components, or by changes in the flotation-oil content. These variations in composition lead to undesired temperature variations in the autoclave which complicate the earlier system and prevent the meticulous control of the autoclave temperature which is required.